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Lecture # 22

Lecture # 22. Higher order visual processing 4 /18/13 With acknowledgement to Prof Dan Butts. Wiki assignments. Introductory page is due at midnight Get the first “ page ” of your project up on the wiki Can be a linked subpage or can be a section on the main page 500-1000 words

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Lecture # 22

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  1. Lecture #22 Higher order visual processing 4/18/13 With acknowledgement to Prof Dan Butts

  2. Wiki assignments • Introductory page is due at midnight • Get the first “page” of your project up on the wiki • Can be a linked subpage or can be a section on the main page • 500-1000 words • Figures or links to make it interesting • REFERENCES

  3. The return of the center-surround Cones are wired up so one cone is compared with one or more surrounding cones The center and the surround cones send inputs to a ganglion cell

  4. Ganglion cells provide outputs to the combined cone inputs Ganglion cell compares the inputs and provides an output Same cones can be wired to different ganglion cells: center + / surround - and center - / surround +

  5. Center - surround color opponency Two cones provide input to 4 channels Red ON green OFF Red OFF green ON Green ON red OFF Green OFF red ON Webvision

  6. Center surround types – receptive field These show an area of the retina and its relative sensitivity to stimulation by a point of light Kuffler’s Nobel prize winning work

  7. Center surround demo

  8. Ganglion cell simulationsDr Jon Krantz – Hanover College

  9. But stimuli are not often dots

  10. Larger scale objects are detected by the edge

  11. What about color? • What if white dot were replaced by red dot or green dot? • What if dots were large

  12. We don’t often look at tiny 2 um spots • What about a large block?

  13. What about other colors? 1 2 3

  14. Visual image

  15. What the photoreceptors see is made of dots

  16. What the brain sees are objects Umbrella Hat Tree Face

  17. Information bottleneck Information flow from retina to cortex Visual cortex 200M cells 1/3 human cortex 1/2 monkey cortex Retina 6.5 M cones 120 M rods 1.2 M ganglion cells Lateral geniculate nucleus (LGN) 2 M cells

  18. RGC respond to edges or annuli

  19. Neuroscience Purves et al eds. Fig 11.2

  20. Wiring to visual cortex • Topographic map is retained • Optic chiasm • Right side from both eyes to right visual cortex • Left side from both eyes to left visual cortex Wolfe et al 2006 Sensation and perception

  21. Neuroscience Purves et al eds. Fig 11.2

  22. Two key ganglion cell types • β (midget cells) • High acuity – one to one wiring of cone to bipolar to ganglion cell • Color vision • α (parasol cells) • Lower acuity – many to one wiring of cones to ganglion cells • Best for motion detection

  23. α and β ganglion cell receptive fields: change across retina

  24. LGN • Relays ganglion cell output • Same center / surround wiring as in retina RGC receptive LGN receptive field field

  25. Lateral geniculate nucleus • geniculate = bent • Layered • Bottom two layers have larger cells • Magnocellular • Magno- = large • Top four layers have smaller cells • Parvocellular • Parvo- = small

  26. Lateral geniculate nucleus • Parvocellular layer receives input from β ganglion cells (midget cells) • High acuity • Magnocellular layers receive input from α ganglion cells (parasol cells) • Motion detection

  27. LGN layers • Topographic mapping • Eyes and fields within the eyes • 1,2 Magnocellular (motion) • 3-6 Parvocellular (color) Layers of LGN

  28. Lateral geniculate nucleus is a relay station between retina and cortex Webvision

  29. Wiring to visual cortex • Cortex feeds back onto LGN • More input from cortex than from retina!! • Inputs from other parts of brain • When sleep, LGN and rest of thalamus shuts down so no signals go to cortex Wolfe et al 2006 Sensation and perception

  30. Wiring to visual cortex • Cortex feeds back onto LGN • More input from cortex than from retina!! • Hard to study • Under anesthesia, link to cortex is shut off Wolfe et al 2006 Sensation and perception

  31. Wiring to visual cortex Topographic map set up in retina, relayed through LGN and passed to cortex Cortex also has 6 layers LGN projects to layer 4 (V4) Magnocellular4cαParvocellular4cβ

  32. Wiring to visual cortex • Cortical magnification • Fovea gets more neurons than periphery • Index finger demo Wolfe et al 2006 Sensation and perception

  33. Primary visual cortex, V1 = striate cortex

  34. Arranged in layers As move from top to bottom, cells are arranged in columns with different sensitivities Orientation Spatial frequency Ocular dominance - prevalence of one eye Color

  35. David Hubel and Torsten Wiesel

  36. Huber and Wiesel • Record from visual cortex while illuminating cat retina • Illuminated retina with spots as their mentor Kufflerhad done in his work to record from retinal ganglion cells • Got nothing

  37. Hubel, 1981 Nobel prize lecture “Our first real discovery came about as a surprise. We had been doing experiments for about a month.. and were not getting very far: the cells simply would not respond to our spots and annuli. One day we made an especially stable recording… For 3 or 4 hours we got absolutely nowhere. Then gradually we began to elicit some vague and inconsistent responses by stimulating some where in the midperiphery of the retina. We were inserting the glass slide with its black spot into the slot of the ophthalmoscope when suddenlty over the audiomonitor the cell went off like a machine gun. After some fussing and fiddling we found out what was happening The response had nothing to do with the black dot. As the glass slide was inserted, its edge was casting onto the retina a faint but sharp shadow, a straight dark line on a light background. That was what the cell wanted, and it wanted it, moreover, in just one narrow range of orientations.”

  38. LGN center surround converted to edge detectors Same as ganglion cells

  39. Orientation detection • Hubel and Wiesel discovered cells that were sensitive to bars • Some responded to orientation

  40. Orientation sensitivity changes with penetration through cortex

  41. Stimulus size match to cell response area (complex cells) Get higher response if stimulus matches the cell’s receptive area

  42. Striate receptive fields

  43. Factors affecting cortical response • Bar width • Bar orientation • Cell type • End stopping • Edge- and bar-detectors • Receptive field size • Motion • Color • Ocular dominance:

  44. Eyes start to come together in cortex • Alternating columns are associated with each eye • As if locating inputs more closely so they can be compared

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